This aerodynamic flow is commonly referred to as the “slipstream”.
While a rotorcraft is flying in translation, the slipstream strikes the tail fin and/or stabilizers that occupy respective positions that are substantially vertical and horizontal relative to the rotorcraft, and that are generally secured to the rear end of the fuselage known as the tail boom in the art.
This impact leads to vibration, commonly known as “tail shake” in aviation technology. The vibration is then transmitted to the rotorcraft as a whole and presents numerous drawbacks, and in particular:                for the comfort of the crew and passengers;        for the fatigue of parts and equipment; and        for the operation of weapons systems of the rotorcraft, if it has any.        
U.S. Pat. No. 3,181,815 discloses a cap disposed on the rotor head. While flying in translation, the cap creates aerodynamic lift and, as a result, deflects the slipstream downwards, mainly against the tail boom and no longer against the tail stabilizer and fin of the rotorcraft. The tail shake effect is greatly diminished.
Nevertheless, starting from a relatively steep angle of attack of the rotorcraft known as the critical angle of attack, the boundary layer separates from said cap. It is also known that when the boundary layer separates from a moving body, there is a sudden loss of lift on the body. Under such conditions, the lift of the cap decreases, the beneficial effect of the cap is attenuated, and possibly even eliminated.